It turns out to have been part of a wake-up call. Before leaving Paris, Kardashian sent a text to longtime friend and celebrity trainer Gunnar Peterson, and no sooner than having landed in Los Angeles, he hit the ground running with a new workout plan.

"I think Rob knows exactly what to do. He's worked out [before] and he's focused," says Peterson, who has previously trained Kardashian, 27, and set up a weekly plan for his weight-loss goals.

"We're doing intervals with strength movements: squats, the sled, swings and push presses. It's about getting his heart rate up and keeping it up."

The goal for Kardashian is to love what he sees when he wakes up every morning.

"I don’t have a scale, I'm not big on measurements. Until a person looks in the mirror and is happy with themselves, you have not reached your fitness goals," says the trainer, who also counts Bruce Willis and Sylvester Stallone as clients. And consistency is front and center: "The key to anyone's success is staying with it!"

Though Kardashian has a busy schedule, he is making time for fitness. "The toughest thing is to fit it in," Peterson says. "People forget he's in the show, he has the sock business and he has other interests. But with a good diet, the right sleep and steady workouts, he will see changes. He'll start seeing and feeling it in a couple of weeks and will see major changes in six to eight weeks."

"I'm a firm believer in trying to make something that might not be considered fun by a lot of people, fun. And hopefully that makes the person have a positive association with it and they want to come back," Peterson says. "Rob works out in beast mode. When I see him, we laugh, have fun and he gets after it hard at the gym. I know he can do this."